Sen. Arlen Specter’s defection to the Democrats sent shockwaves through Washington that will be felt for months, if not years, to come.

It’s a good day for President Obama, a wake-up call for Republicans, and a bad day for American centrists who believe in checks and balances. Because with Specter and the likely seating of Al Franken, Democrats will reach a filibuster-proof 60-seat majority.

For President Obama, this action on the eve of his 100th day in office reinforces his aim to build a realigning center-left coalition. The groundwork was laid by moderates' and independents' rejection of the Bush administration’s red vs. blue play-to-the-base politics. Obama’s patented “not red states or blue states but the United States” appeals enabled him to win virtually every swing state in the 2008 election. But when a respected five-term Republican Senator of one of those states, Pennsylvania, jumps ship—well, that’s a sign of a swing state becoming Democratic and a once-in-a-generation realignment under way.

Before Specter’s defection is automatically written up as a new chapter in
Profiles in Courage, there is reason to question whether the braver, better course of action would have been to stick out another primary fight as a Republican.

But it’s not all due to President Obama’s broad popularity outside the conservative base. It’s due to Sen. Specter’s increasing isolation in the Republican Party. Centrists have been forced to the margins in the Party of Lincoln, even as the party itself has been forced to the margins of American politics. The two dynamics are directly connected.

A decade ago, Republican congressmen dotted the party’s historic home of New England. Now there are none. The trend is continuing across the Northeast, exacerbated not only by Bush-era red-state policies that alienated the moderate majority of voters, but by the party’s penchant for pitting conservative primary challengers against centrist incumbents that ended up aiding only the Democrats. Specter offered up a litany of such self-inflicted losses, from Lincoln Chafee to Wayne Gilchrist to Heather Wilson. “They don’t make any bones about losing elections so long as they purify the party,” he said. “I don’t understand it…There ought to be an outcry.”

Specter was facing another primary challenge from the leader of one such heretic-hunting group, Club for Growth chairman Pat Toomey. The conservative calls for Specter’s head turned to howls when he worked with Maine Republican Sens. Susan Collins and Olympia Snowe along with 16 Democratic centrist senators to cut $118 billion of pork from the stimulus bill, enabling its ultimate passage. Seeing the absurdity of the centrist Catch-22, and motivated by his own political survival, Specter realized that he had a better chance of winning a general election than a low-turnout, hard-core partisan primary. And so he leaped.

But before Specter’s defection is automatically written up as a new chapter in
Profiles in Courage, there is reason to question whether the braver, better course of action would have been to stick out another primary fight as a Republican. Specter had the support of the National Republican Senatorial Committee, which understood that his primary victory was the GOP’s best hope of holding on to the seat. Independent voters appreciate Madison’s vision of checks and balances—that’s why they so often split their tickets, especially in states like Pennsylvania. But the aim of checks and balances—so vociferously defended by Democrats when Republicans tried to invoke the so-called nuclear option over judicial confirmations in 2005, and salvaged by the centrist Gang of 14, including Specter—Is now under threat from the left. Don’t expect many Democratic warnings of the same dangers. In Washington, where you stand is a matter of where you sit. Partisanship trumps principle.

Now the hope for checks and balances will lie almost exclusively with the newly formed Centrist Democrat Coalition, led by Evan Bayh, with whom Specter might logically caucus. Specter took pains to repeatedly say “I will not be an automatic 60th vote” in his press conference and seems to identify most with independent Sen. Joe Lieberman as a political soul mate. He is essentially declaring independence from the insanity of hyperpartisan primaries, wanting to be judged by a jury of all his fellow state citizens.

Today, Specter not only confirmed but compounded the Republican Party’s increasingly isolated right-wing activist tilt. By jumping while he was being pushed, he has undercut the already besieged centrist Republican tradition he tried to steadfastly defend for 29 years. He has reinforced as well as reacted to the dynamic he despises—“the extremes of both parties are taking over.”

John P. Avlon is the author of
Independent Nation: How Centrists Can Change American Politics. Avlon was director of speechwriting and deputy director of policy for Rudy Giuliani's presidential campaign. Previously, he was a columnist for the New York Sun and served as chief speechwriter for then-Mayor Giuliani.